The present invention relates to a novel coating process that comprises the use of a functional photoinitiator, or a macroinitiator derived therefrom, in a cascade of process steps, wherein, on the one hand, a functional photoinitiator or a macroinitiator derived therefrom is covalently bonded to a carrier and, on the other hand, an oligomer or polymer forming a new surface layer is covalently bonded to the functional photoinitiator, or to the carrier modified by a functional photoinitiator, via functional groups that are co-reactive with isocyanate groups. The invention relates also to novel intermediates that are carriers to which functional photoinitiators that contain free isocyanate groups are bonded.
The surface-modification of polymers has been the focus of interest for many years. The properties of a polymer often have to satisfy various physical and chemical requirements which, because of the material used, can often be met only in part. One possible way of meeting those requirements is to cover a base material with a thin layer of a second material. The latter should supply the properties lacking in the first material but not alter the fundamental properties of the base material. Special attention is paid in this connection to improved bio-compatibility in the widest sense, for example to the wettability of polymer surfaces.
A pioneering work in the field of surface-modification of polymers, aimed at improving specifically the wettability of the surfaces of contact lenses, has come from Yasuda et al., J. Biomed. Mater. Res. 9, 629 (1975). The authors describe a process in which a layer of 20 nm thickness is applied by plasma polymerisation of a mixture of acetylene, water and nitrogen to a PMMA contact lens. The plasma is produced in an apparatus for downstream coating, with a high-frequency glow discharge and an operating frequency of 13.56 MHz. The contact angle of a normal, untreated PMMA surface is, according to the standing water droplet method, approximately 71.degree. and, after the plasma polymerisation coating just described, approximately 37.degree..
Another attractive method for applying thin hydrophilic films to substrates is to use unsaturated alcohols in the plasma polymerisation. Hozumi et al., Pure & Appl. Chem. 60, 697 (1988), describe a high-frequency glow discharge method in which they employ allyl alcohol and propargyl alcohol, and also 2-methyl-3-butyn-2-ol. The tests, carried out mainly with propargyl alcohol, show that a contact angle of 45.degree. results after the coating. If, in addition, water is added to the operating gas in the plasma polymerisation, the contact angle can even be reduced to 20.degree.. When a product so coated is swelled with water, however, it is found that the additional layer exhibits unsatisfactory adhesion to the substrate.
Two more recent publications, PCT-AU 89/00220 and H. J. Griesser, Materials Forum, 14 (1990) 192, deal with a plasma polymerisation method in which organic compounds, such as saturated alcohols, saturated amines, derivatives or mixtures thereof, and inorganic gases, such as oxygen, hydrogen, nitrogen, helium, argon or neon, and water vapour are applied as a plasma polymer to a contact lens. According to the authors, the water content should lie between a maximum of 20% by volume and preferably 5% by volume. The presence of water is intended to prevent excessive crosslinking of the plasma polymer. Examples are given in which films are applied by plasma polymerisation of ethanol and isobutanol. During the glow discharge, the substrates are subjected to an energy of approximately 1 watt/cm.sup.2 between two plane-parallel electrodes. If a sufficiently high static potential occurs on those substrates, spontaneous, high-energy discharges are released which greatly heat the substrate and cause internal stresses. As a result, plasma polymer deposits that are highly crosslinked and difficult to control are produced.
WO 94/06485 describes a multi-layer material, especially a biomedical article and preferably a contact lens, having one or more wettable surfaces capable of holding an intact film of aqueous fluid, the multi-layer material consisting of a base material and a hydrophilic layer, and the hydrophilic layer being formed by a carbohydrate derivative that is covalently bonded to reactive groups on the surface of the base material either directly or indirectly via functional groups of a further oligofunctional compound which is laid between the base material and the hydrophilic layer and covalently bonded on both sides.
By the specific use of suitable functional photoinitiators in a multi-stage coating process it has now become possible accurately to control the nature of the applied layers by different reaction mechanisms, namely photo-chemical and chemical reaction mechanisms. The use of functional photoinitiators makes it possible to produce firmly bonded surface layers of high uniformity, layer thickness, coating density and durability.
EP-A-632 329 already describes functional photoinitiators that are also employed according to the present invention. According to that prior art, however, the photoinitiators are always used in such a way that first the isocyanate group thereof reacts with co-reactive groups of the substrate surface. The attachment of oligomers or polymers forming a new surface is then effected by way of radicals by means of the photoinitiator part.